1. Field of the Invention
The present invention relates to an optical functional device for use in optical communication, optical information processing, optical interconnections and the like, and in particular relating to an optical functional device such as an optical modulator for converting electric signals into optical signals, variable optical attenuator, and a driving method and a fabrication process of such optical functional devices.
2. Description of the Related Art
Optical functional devices or photonic devices, represented by an optical modulator which converts an electric signal as the modulation signal into an optical signal, an optical attenuator which attenuates an optical signal in accordance with an electric signal that represents the amount of attenuation, are key components that play markedly important roles in association with rapid increase in communication traffic on the recent optical communication market. With the development of optical communication technologies, these optical functional devices such as optical modulators, optical attenuators have been being developed into high performance and reduced cost products. As a result, studies on application of optical functional devices, not only to the optical communication field but also to the fields where a greater number of devices are required, typified by optical information processing, optical interconnections and the like have been started. The optical modulators and optical attenuators are essentially the same from the viewpoint that both change intensity of light wave in accordance with an electric signal though there is a difference in that an optical modulator changes the intensity of light wave at high speeds while an optical attenuator changes the intensity of light wave at low speeds. In this description, an optical modulator is taken as a representative of the optical functional devices. As the optical modulators, electroabsorption modulators using compound semiconductors, Mach-Zehnder type modulators using LiNbO3 (lithium niobate) material have been put to practical use.
The Mach-Zehnder type optical modulator is based on the Mach-Zehnder interference optical system. The Mach-Zehnder interference optical system is essentially composed of an input optical waveguide for receiving a light wave signal, first and second waveguide portions, a splitter connected to the input optical waveguide for separating the light wave signal into two parts and propagating them to first and second waveguide portions, a wave combiner for recombining the light wave signals from the first and second waveguide portions, and an output optical waveguide for outputting the combined light wave signal. The Mach-Zehnder type optical modulator is constructed so as to be able to vary the intensity of the light at the output light, using the Mach-Zehnder type interference effect, by changing the phase difference between the light wave signals that propagate through the first and second waveguide portions.
Since the Mach-Zehnder type optical modulator operates based on the phase difference between the first and second waveguide portions, the intensity of the output light also changes in accordance with the wavelength if the wavelength of the input light is varied. Accordingly, there have been also proposed tunable filters (i.e., variable wavelength filters) and others that operate based on the Mach-Zehnder type modulator.
For example, Japanese Patent Application Laid-open No. H8-95108 (JP, 8-095108A) discloses an optical signal processor in which a plurality of unit structures, each including a ring-type waveguide mode-coupled with the first waveguide portion in the basic structure of the aforementioned Mach-Zehnder type interference optical system, are cascaded in multiple stages. In this optical signal processor, the unit structure to be cascaded is designed suitably so that it is possible to construct an optical filter having a desired characteristics. It is also possible to make the device have variable characteristics by providing a phase controller in the waveguide portion. However, in the device described in JP, 8-095108A, the device characteristics sharply change depending on the coupling coefficient (i.e., amplitude branching ratio) between the ring-type optical waveguide and the waveguide portion. Since this amplitude branching ratio greatly varies due to the influence of structural errors and other factors, it is difficult to fabricate and adjust the device so as to satisfy the desirable characteristics.
Japanese Patent Application Laid-open No. 2004-10297 (JP, 2004-010297A) discloses a tunable filter including, in the basic structure of the aforementioned Mach-Zehnder type interference optical system, a first ring-type optical waveguide mode-coupled with the first waveguide portion, and a second ring-type optical waveguide mode-coupled with the second waveguide portion. In this tunable filter, the indexes of refraction are varied by current injection or voltage application to the ring-type optical waveguides, or other method, so as to change the optical lengths of the ring-type optical waveguides, whereby the wavelength to be transmitted is changed. In general, the optical length is represented by a product of the physical length and the refractive index.
In the optical signal processor of JP, 8-095108A or in the tunable filter of JP, 2004-102097A, the intensity of the output light varies in accordance with the electric signal if the wavelength of the input light is constant.
As an example of electroabsorption modulators which use compound semiconductor, a modulator integrated light source using InP material has been disclosed by Ishizuka et al. [M. Ishizuka et al., “Modulator Integrated DFB Lasers with More Than 600-km Transmission Capability at 2.5 Gb/s,” IEEE Photonics Technology Letters, Vol. 9, No. 10, pp. 1406-1408 (1997)]. This modulator integrated light source has been put into practical use as a light source for long-distance optical communication. As an example of Mach-Zehnder modulator using LiNbO3 material, a miniaturized device has been particularly reported by Sugiyama et al. [M. Sugiyama et al., “Compact Zero-Chirp LiNbO3 Modulator for 10-GB/s Small-Form-Factor Transponder,” 30th European Conference on Optical Communication, Post-Deadline Session 3, Th4.3.5 (2004)]. However, the electroabsorption modulator proposed by Ishizuka et al. suffers from difficulties in reducing cost from the viewpoint of the used wafer material and fabrication cost. Though miniaturized, the Mach-Zehnder modulator proposed by Sugiyama et al. has a device size of 1 cm or greater, so there is a limit to reduce the cost.
A cheaply manufacturable optical modulator using Si semiconductor has been recently disclosed by Ansheng Liu et al. [Ansheng Liu et al., “A high-speed silicon optical modulator based on a metal-oxide-semiconductor capacitor,” Nature, Vol. 427, pp. 615-618 (2004)]. The advantage of use of Si semiconductor resides in that, in addition to availability of matured processing techniques, there is a possibility of development into OEIC (optoelectronic integrated circuit) based on Si wafers, owing to the compatibility of Si with CMOS LSI technology. The optical modulator proposed by Liu et al. was revolutionary in the respect that not only passive optical waveguide portions but also active waveguide portions were formed on a Si wafer. However, the device configuration is to perform no more than conversion of the change in refractive index caused by application of an electric field to the vicinity of a PN junction into light intensity by use of a simple Mach-Zehnder interference optical system, and a device size equal to or greater than 1 cm is needed. So, there is not only a limit to cost reduction from the viewpoint of the yield obtained from one wafer when the device size is on the order of centimeters, but also it is difficult to develop an OEIC configuration by integrating the device with a CMOS LSI or the like. That is, there is the problem that the merit of use of Si semiconductor cannot be put to sufficient use.
Optical functional devices such as optical modulators, optical attenuators and the like are not only applied to the field of optical communication but application to the fields of optical information processing and optical interconnection have started to be discussed and studied. However, in order to make the optical functional devices widespread in the fields of optical information processing and optical interconnection, it is necessary for optical functional devices to endure mass production and it is also necessary to sharply cut off the fabrication cost. It is said that the fabrication cost of the products to be used in the fields of optical information processing and optical interconnection should be cut down two or more digits, compared to the products in the field of optical communication. In view of reducing fabrication cost, it is important to be able to manufacture a downsized device while maintaining the desired modulation characteristics or attenuation characteristics. Also, in order to make the device endure mass-production, it is desired that the characteristics of the device is little affected by structural variations.
However, in the aforementioned conventional optical functional devices, both the optical signal processor proposed by JP, 8-095108A and the tunable filter proposed by JP, 2004-102097A, a propagation delay of a light wave signal is caused by changing the refractive index of a waveguide through which the optical signal propagates so as to obtain variation in the intensity of the output light in accordance with the phase delay resulting from the propagation delay, based on the Mach-Zehnder interference effect. Accordingly, it is theoretically difficult to reduce the size of the conventional optical functional devices. Since largeness in size implies large parasitical electric capacitance, such optical functional devices are hard to operate at high speeds. Since the optical modulator using Si material, proposed by Liu et al. uses the matured semiconductor manufacturing technologies, the devices can be mass produced with stable characteristics; yet it is impossible to achieve downsizing, so there is a limit to cost reduction.